My short answer is "yes!"
$^#& - if you don't want to come to a wedding then don't RSVP. That's what RSVP cards are for! Honestly, I don't mind if you say "no" to the RSVP, but don't say "yes" only to change your mind 2 days be...fore. Most people don't realize (until they've planned a wedding) that the meals/slots are nonrefundable starting the week of... I bet I'm going to cross the "$1k wasted on ghost-guests" threshold today...
It's astonishing to me how many people don't RSVP to events or change their answer at the last minute. This may not be a big deal for a casual affair, but for a wedding or something else requiring planning knowing the guest count is very important to your host. Not responding or changing your response is just plain inconsiderate.
Sure, occasionally an invitation gets lost or circumstances may make you able to attend or not attend at the last minute. If that happens, apologize vehemently! Unfortunately, many people are habitual non-responders or mind changers. If that is you, stop it. You're costing people money. In the case of an event with seat assignments you are also causing stress because adjusting seating charts is a pain in the ass.
I think the level of rudeness increases with the formality of the event. Therefore, these breaches of etiquette are most egregious for something like a wedding. However, most hosts appreciate having an idea of how many people to expect, so it is thoughtful to respond accuarately to evites and Facebook announcements as well.